To meet industry needs for electrical power converters that convert AC to DC without injecting large amounts of harmonic currents into the power system, various patented circuit topologies are available, as described in the cited publications.
Eighteen-pulse converters have found widespread acceptance in the application of variable frequency drives, and the search for further cost reduction continues. Also, means to obtain higher pulse number converters such as 24-, 36-, and 48-pulse are sometimes required.
In some applications it is advantageous to construct a multipulse converter by paralleling multiple 3-phase bridge converters, such as are provided by a 3-phase bridge rectifier. Paralleling is relatively straightforward if a full-rating double-wound transformer is used. However, significantly lower cost can be achieved with autotransformers in conjunction with ZSBTs (zero-sequence blocking transformers). This procedure is taught in U.S. Pat. No. 6,101,113. It is desirable that the ZSBTs exhibit high magnetizing inductance so that the minimum DC load to prevent high DC output voltage is not excessive.
The autotransformer patent method described here not only eliminates the ZSBTs and reduces the amount of DC load to restrict the open circuit output voltage, but it can be applied to any multipulse configuration.